Computer systems and portable electronic devices such as tablets and smart phones require large capacity, high speed volatile and non-volatile storage of data. Present types of semiconductor memory that use a combination of specialized volatile and non-volatile technologies. One type of volatile memory is static random-access memory (SRAM) which is often implemented using a bistable transistor flip-flop or a latching circuit. The word “static” indicates that the memory retains its contents as long as power remains applied. “Random access” means that locations in the memory can be written to or read from in any order, regardless of the memory location that was accessed last. SRAMs offer advantages including reliability and fast reading and writing of the stored data, however the data retained in the SRAM cell is volatile. Interruption of the power supply source causes loss of the data in the SRAM cell.
A non-volatile SRAM (nvSRAM) includes an SRAM cell coupled with two or more non-volatile memory transistors to store data written to the SRAM cell in the event of an interruption of power. The non-volatile memory transistors may be implemented in different ways, for example using a Silicon-Oxide-Nitride-Oxide-Silicon (SONOS) transistor or a floating gate transistor in which a stored charge modifies the transistor's threshold voltage.
One disadvantage of present nvSRAM circuits is their limited density and relatively large memory cell size, typically including 12 or more transistors per nvSRAM cell.
Thus, there is a need for a non-volatile latch which overcomes the shortcomings of the conventional NVL cell architecture.